The applicant of the present invention has pursued development of techniques for modifying self-fluxing pellets to be used as an iron raw material for a blast furnace from the 1970s to 1980s and completed development of the techniques with which self-fluxing pellets (self-fluxing dolomite pellets) having good reducibility at high temperature (hereinafter referred to as “high-temperature reducibility”) can be manufactured by blending, as CaO and MgO sources, limestone and dolomite with iron ore such that the resulting blended raw material has a CaO/SiO2 mass ratio of 0.8 or more and a MgO/SiO2 mass ratio of 0.4 or more, pelletizing the blended raw material into raw pellets, and burning the raw pellets (Refer to Patent Documents 1 and 2).
The applicant of the present invention has also pursued development of burden distribution control techniques for blast furnaces concurrently with the development of techniques for modifying the self-fluxing pellets, and has completed development of center coke charging technologies that can dramatically improve air and liquid permeabilities in blast furnaces (refer to Non-Patent Document 1).
The use of the self-fluxing dolomite pellets and application of the center coke charging techniques have made it possible to stably and efficiently produce pig iron in blast furnaces that use both pellets and sintered ore as the iron raw material with large quantities of pulverized coal injected into the furnaces.
The self-fluxing dolomite pellets (may be simply referred to as “self-fluxing pellets” or “pellets” hereinafter) have a CaO/SiO2 mass ratio (abbreviated as “C/S”) and a MgO/SiO2 mass ratio (abbreviated as “M/S”) adjusted to particular values or higher by adding limestone and dolomite as the auxiliary raw materials to the iron ore; however, the amounts of limestone and dolomite blended are desirably reduced as much as possible to reduce the cost of manufacturing the pellets.
In order to meet the recent rapid increase in steel demand, the production of pig iron needs to be increased further. For blast furnaces that use both sintered ore and pellets as the iron raw material, pellets that have better high-temperature reducibility and that can further increase the productivity under high-level coal injection operation are desirably provided.
According to the knowledge subsequently gained by the applicant, it has been found that the high-temperature reducibility of the self-fluxing dolomite pellets is not solely determined by defining C/S and M/S but is also in no small measure influenced by the iron ore grade of the pellets (i.e., the iron grade of the iron ore used). In other words, it has been found that the optimum combination ranges for C/S and M/S vary according to the iron ore grade of the pellets.
However, the quantitative determination of the degree of such an influence has not been systematically investigated so far, and little is known about a more suitable C/S and M/S combination range that takes into account the iron ore grade of the pellets.    Non-Patent Document 1: Matsui et. al, “Blast Furnace Operational Technology and Central Gas Flow Intension for Center Coke Charging at Kobe Steel”, R&D Kobe Steel Engineering Reports, Vol. 55, No. 2, September 2005, pp. 9-17    Patent Document 1: Japanese Examined Patent Application Publication No. 3-77853    Patent Document 2: Japanese Examined Patent Application Publication No. 3-77854